The invention relates to aircraft towing vehicles.
A previously tested prototype of a towing vehicle of the general type under consideration herein, which was reported on in the media, has a hold-down mechanism or holder for the aircraft nose wheel defined by horizontal arms. Before a nose wheel can be engaged these arms must be positioned according to the nose wheel diameter of the aircraft type that is to be towed such that the apex of the nose wheel contacts the holder to thereby secure the nose wheel to the vehicle. A lifting mechanism for the nose wheel, which initially engages the nose wheel and moves it into its secured position, is mechanically locked in a predetermined location. By properly prepositioning the horizontal arm, the nose wheel is secured to the towing vehicle at the end of the engaging operation. Such a towing vehicle has several disadvantages.
The height adjustment of the holding mechanism, which may be performed with the help of a computer which stores the necessary positioning data and after the aircraft type has been punched in (to determine the applicable nose wheel diameters), is a source of potential error. Further, the lifting and holding mechanism of such towing vehicles secures the nose wheel solely by engaging the nose wheel periphery with fixed surfaces without exerting a positive clamping force against the wheel. That is, the nose wheel is secured by virtue of the relative fixed positions of the lifting mechanism and the horizontal arm and not with a movable member capable of following the wheel into engagement and effectively resiliently applying a holding force to the wheel so that, for example, minor variations in the wheel diameter, say due to wear, do not affect the force with which the wheel is held.
This failure to pressure-lock the nose wheel into its secured position can lead to relative wheel movements as the aircraft is being towed, particularly at the start of the towing operation when the wheel is subjected to movements resulting from a pitching of the aircraft because the inherent resiliency of the tires allows a certain amount of play even after the nose wheel has been secured. Such relative movements can increase in amplitude, they can overstress the nose wheel undercarriage or, worse still, they can work the nose wheel loose from the hold-down mechanism.
In contrast to conventional, low speed towing vehicles which employ a towing bar coupled to the nose wheel undercarriage towing vehicles which grip the entire nose wheel were intended to form a much more stable and secure connection between the aircraft nose wheel and the towing vehicle. Such towing vehicles are intended to operate at much higher speeds and, for example, they are to tow fully loaded aircraft to and from the runway. They must, therefore, be constructed in such a way as to preclude the possibility of the nose wheel becoming disengaged from the towing vehicle while the aircraft is being towed.
German patent publication No. 35 34 045 discloses a towing vehicle of this general type, although it was never actually built. It has a movable lifter and an associated hold-down mechanism and, once the nose wheel has been engaged the two are also mechanically or hydraulically locked, but not pressure clamped, in position.